Using Optical Coherence Microscopy to Study Early Frog Development

We have used the non-invasive and non-destructive imaging technique of optical coherence microscopy (OCM) to document morphological changes that occur internally within the frog Xenopus laevis during gastrulation, a critical event in the early embryonic development of animals. OCM generates images by using interferometry to penetrate deeply within turbid tissue to measure inherent cellular light scattering properties. We have generated two-dimensional and three-dimensional time-lapse movies that depict in vivo motion that is consistent with the behavior of the internal migrating mesoderm during gastrulation. This achievement is noteworthy because mesodermal migration occurs at depths prohibitive to conventional light, two-photon and confocal microscopy, and also because there are no published accounts of gastrulation within a single embryo. Light scattering theory predicts that a light source with a longer wavelength will allow for deeper scans. To verify this, we have compared images of developing embryos acquired using Harvey Mudd College’s three-dimensional OCM (λ = 843 nm) to images acquired using a similarly designed two-dimensional microscope (λ = 1310 nm) at the Beckman Laser Institute. We have found that the longer wavelength indeed provides an enhanced depth penetration. We have also characterized the optical scattering properties of the ectoderm and mesoderm by determining how quickly the OCM signal attenuates within each tissue layer. The signal attenuations in the ectoderm (18.3 ± 3.0 mm-1) and mesoderm (19.2 ± 5.0 mm-1) agree within uncertainties, suggesting that these tissues are optically indistinguishable, at least in terms of bulk optical properties. Moreover, the mean value for both tissues at 1310 nm is 14.0 ± 2.0 mm-1 (compared to 19.0 ± 3.0 mm-1 at 843 nm) leading to a 36 % improvement in depth penetration at the longer wavelength.